Telephone and utility service providers frequently inspect or repair lines, trees, and other objects located at elevated heights. Boom arms fitted with baskets are commonly mounted to vehicles for elevating personnel carried within the basket. Boom arms for such vehicles can be constructed in a variety of configurations, including, for example, an over-center boom arm that can unfold from a horizontal position to a vertical position.
Boom arms are typically hollow tubes that are strong and lightweight with a multi-layer construction. One type of boom arm has an inner metal layer bonded to an intermediate composite layer (e.g., a steel portion that extends 10 to 14 inches across a connection point between fiberglass portions). An outer layer is constructed of a protective material, such as a gel-coat, and is bonded or applied over the composite layer.
The metal layer and the composite layer have different stiffnesses. To provide a smooth transfer of bending stresses created by the load in the basket from the composite layer to the metal layer, the end of the metal layer is tapered over a region around the inner circumference of the boom arm. The tapered region allows a band of stress between the metal layer and the composite layer to dissipate. For example, the tapered region diffuses the stress into a band having a width of about six to ten inches. Without the tapered region, the stress would form a stress line, increasing the likelihood of failure of the composite layer.
The metal layer, and particularly the tapered region of the metal layer, is subject to degradation by, for example, corrosion or wear. When corrosion occurs, rust is produced and the thickness of the metal material at the tapered region is reduced. Because the production of rust does not occur uniformly, the remaining material at the tapered region forms into peaks and valleys, increasing the magnitude of stresses at stress points, rather than across a band. Rust is also worn into the composite layer adjacent to corrosion spots in the metal layer, eroding the composite material and reducing the strength of the composite layer. Finally, as metal and composite material at the tapered region is depleted by degradation, gaps form between the composite layer and the metal layer, reducing the generally uniform transfer of stresses at the tapered region.
Each vehicle-mounted boom arm can be subject to different environmental conditions depending on the use of the boom arm and the local climate. As a result, it is difficult to predict if and when degradation such as corrosion and wear will occur. Furthermore, because degradation occurs on the inside of the boom arm, there may not be any indicators of corrosion, erosion, wear etc. on the exterior or visible surface of the boom arm. In order to access the interior of the boom arm for examination, the boom arm would have to be disassembled or even destroyed with certain boom configurations.